Field of the Invention
This invention relates to the field of wireless communications earplugs, magnetic field antennas and radio frequency identification devices.
Description of Related Art
Aircraft radios and intercoms typically provide electrical audio outputs for connecting aviation headsets. The audio signal from an aircraft radio, provided as an electrical signal at an audio output, can directly drive an aviation headset that incorporates headset speakers with enough sensitivity to provide clear acoustic audio communications.
Published applications US 2009/0041285 and US 2010/0296667, of which this application is a continuation-in-part, describe components of a system that can be used for wireless earplug communications. In one embodiment described in US 2009/0041285, a magnetic field antenna (MFA) may be used as a battery-free wireless communications earplug in which only passive electrical components are employed. The sensitivity of the antenna achieves adequate sound pressure level (SPL) and perceived acoustic volume with typical aircraft radios, and the small earplug size provides a comfortable earplug.
A pair of magnetic loop transmitters, such as a loop transmitter that fits around the pinna of the user as seen in at least FIG. 12 of US 2009/0041285, can be installed in a headset instead of the speakers, or used in conjunction with the speakers. The audio electrical signal output from the aircraft radio can drive these loop transmitters in a headset and be received by the battery-free communications earplugs of US 2009/0041285 and US 2010/0296667 with enough system sensitivity to drive the speakers of the earplugs to achieve high enough SPL for clear speech communications and high speech intelligibility, while providing an earplug design that fits in a human ear.
Moreover, electromagnetic shielding of electrical components used in wireless earplugs is important to prevent pickup of spurious fields. However, the antenna must be small enough to fit comfortably in the human ear. Large communications earplugs tend to be uncomfortable because they put pressure on the ear. Moreover, a large earplug may interfere with a headset earcup when the earplug and headset are worn together to provide “double hearing protection”. A higher sensitivity MFA device can achieve the same sensitivity as a lower sensitivity MFA but in a smaller package size.
It is known in the art that an antenna coil can be constructed using a helical coil of wire with at least one turn. According to Faraday's Law, also well-known in the art, a time-varying magnetic field passing through the inner region of a coil will produce a voltage on the ends of the coil proportional to the time derivative of the magnetic flux through the turns. The magnetic flux through a coil is proportional to the density of magnetic field lines passing through it. When an antenna coil is used to generate a voltage from a magnetic field, it is called a receiver coil. When an antenna coil is used to generate a magnetic field by applying a voltage or current to the coil, it is called a transmitter coil.
Electromagnetic waves impinging on electrical components can produce undesired voltages and currents that cause undesired effects. The source of the electromagnetic waves may be a remote communications transmitter, the transmitter antenna of an embodiment of the invention or they may be stray electromagnetic waves generated by other equipment. It is undesirable for the electrical components used within the antenna to generate voltages from external fields in an unpredictable way. Even connection wires benefit from electromagnetic shielding. In particular, electret microphones are sensitive to electromagnetic fields, and this is a known problem in the art.